The invention relates to a process for ablating small holes in to multiple layers of material and particularly to a method of preventing deformation of a layer of material into which holes are drilled via an Excimer LASER.
The use of ablation patterning of various polymeric materials, e.g., polyimides, is known. U.S. Pat. No. 4,508,749, for example, disclosed the use of ultraviolet (U.V.) radiation for etching through a polyimide layer. This patent is primarily directed to producing tapered openings through a polyimide layer for exposing surface areas of an underlying layer of metal. Electrical connections are then made through the openings to the metal layer. U.S. Pat. No. 5,236,551 likewise disclosed ablation etching for patterning a polymeric material layer which is then used as an etch mask for etch patterning, using wet or chemical etchants, an underlying layer of metal.
In a typical ablation process, a beam of laser energy is directed against an exposed surface of a body to be ablated. The laser energy is absorbed by the material and, as a result of photochemical, thermal and other effects, localized explosions of the material occur, driving away, for each explosion, tiny fragments of the material. The process requires that significant amounts of energy be both absorbed and retained within small volumes of the material until sufficient energy is accumulated in each small volume to exceed a threshold energy density at which explosions occur.
Polymeric materials, such as polyimides, are well suited for use in the process because such materials have a high absorptivity for U.V. light while having a relatively low thermal diffusivity for limiting the spread of the absorbed energy away from the volume where the energy was absorbed. Thus, the energy level quickly builds above the required energy density threshold.
When an excimer laser is used, because of the unique optical focusing requirements of the excimer laser it is important to the manufacturing process that the material to be ablated be flat, with a typical peak-to peak roughness of less than about 20 microns, i.e., xc2x110 microns for a given ablation operation. This need and others are addressed by the instant invention.
The present invention is directed toward improving the results of a laser ablation process of a thin film in which holes are ablated through the film. A method of ablating a film according to the present invention involves placing the thin film under such conditions to cause it to come under tension so as to remove wrinkles that may be present in the film, thereby orienting the surface of the film substantially in a single plane to allow precise and accurate ablation of multiple holes in the film simultaneously by applying laser energy to it.
In one example, a thin layer is laminated with an additional layer to form a laminate under conditions comprising a first temperature, and then pulses of LASER energy are directed at the laminate, under conditions comprising a second temperature, in a manner which results in holes being drilled in the thin layer. The first temperature is different from the second temperature and the thin layer has a coefficient of thermal expansion different from a coefficient of thermal expansion of the additional layer, such that after laminating at the first temperature, the additional layer places the thin layer under tension at the second temperature.
The first temperature is greater than the second temperature when the coefficient of thermal expansion of the additional layer is less than that of the thin layer, and vice versa. The thin layer preferably comprises polyimide, while the additional layer can be made of various different materials. However, the principles of the present invention apply equally to thin layers other than polyimide.
When the second temperature is higher than the first, the second temperature may be achieved, at least partially, through absorption of laser energy by the additional layer. Heating or cooling during the ablation process can be achieved or supplemented by heating or cooling an ablation chuck, which interfaces the laminate during the ablation process.
A method of preparing a laminated material for laser ablation is provided, which includes laminating a first layer to a second layer, wherein the first and second layers have different coefficients of thermal expansion, and wherein the second layer has an interior hole such that when the laminated substrate is formed, a region of the first layer aligned with the hole is not laminated to the second layer and is surrounded by laminated regions.
A laminated material is provided, which includes first and second layers, wherein the first and second layers have different coefficients of thermal expansion, and wherein the second layer has an interior hole such that a region of the first layer aligned with said hole is not laminated to the second layer and is surrounded by laminated regions.